1. Field of the Invention
The invention relates generally to polymeric blends having high impact strength and, more particularly, the invention relates to blends of thermoplastic polypropylenes and butene polymers, which exhibit improved resistance to stress whitening or bruising.
2. Description of Related Technology
Many applications incorporating polymeric materials, including batteries, furniture and vehicle parts, require high impact strength, stiffness, and toughness. Useful materials generally have unfilled flexural modulus values between 100,000 and 250,000 psi. However, impact polymers having the desired strength and stiffness typically suffer from cosmetic defects (such as discoloring) caused by external stresses, generally described as stress whitening. This stress whitening can result in cosmetic rejection of a finished product and in some applications the problem is significant. For example, in injection molding processes, the molded article is ejected from the mold using ejection pins. The removal process unavoidably stresses the product at the points of contact with the pins, resulting in cosmetically unacceptable discolorations.